Parallel connection of panels with very different sizes

[Sverige]

I have a question about connecting panels in parallel. I understand their capacities are added to one another and if they have similar characteristics (peak voltage?) then there’s no downside to doing it.

But what if one panel is shaded, does it degrade the performance of the panel which is in full sun?

I have one 160W poly panel (specs below) connected to a 20A PWM controller charging a pair of car batteries(100Ah +55Ah in parallel). If I add two 5W panels (tiny by comparison) in parallel, will they add a small amount to the performance of the 160W panel at no risk of reducing the output of the larger panel? If the smaller panels becoming shaded (bird poop, etc) risks dragging down the voltage from the 160W panel, then I guess I shouldn’t do it.

In that case, is it better to connect the tiny panels direct to my car batteries (omitting the PWM controller), as they are small enough not to damage the batteries with overcharging.

Specs of the larger 160W panel and the pair of 5W panels below. I realise the 5W panels won’t contribute much extra power, but something is better than nothing, right?!

[Technical data] Rated power (Pmax): 160 W - max. Voltage (Vmp): 18.1V - Max. Current (Imp): 8.85A -- Open circuit voltage (Voc): 22.32V -- Short circuit current (Isc): 9.56A -- Max. System Voltage: 1000VDC -- Operating Temperature : -45°C to +85°C -- Dimensions: 1460*664*35 mm -- Weight: 12kg

Technical data: rated power (Pmax): 5 W, max. Voltage (Vmp): 18.10 V, max. Current (Imp): 0.29A -- Open circuit voltage (Voc): 22.30V -- Short circuit current (Isc): 0.30A -- Max. System Voltage: 1000VDC -- Operating Temperature : -45 °C to +85 °C, dimensions: 230 x 185 x 17 mm, weight: 1.00 kg.

 

[gnubie]

In parallel shading makes no appreciable effect on the other panels. In parallel your voltages must be similar, the closer the better. Voltage mismatch is not so much of a problem with PWM but closely matching is still a good idea.

If you have grossly mismatched panels, in terms of current rating, in parallel you must always use appropriate fuses between the panels and the common wiring.

 

[Sverige]

In parallel shading makes no appreciable effect on the other panels. In parallel your voltages must be similar, the closer the better. Voltage mismatch is not so much of a problem with PWM but closely matching is still a good idea.

If you have grossly mismatched panels, in terms of current rating, in parallel you must always use appropriate fuses between the panels and the common wiring.

Thanks for your reply, I really appreciate it. Sounds like I need not worry about the smaller panels dragging down the larger one if they become shaded.

Could you elaborate on the need for fuses when panels are mismatched (as is the case with my proposed 160W + 2x5W setup)? I don’t understand what the reason for this might be. Thanks.

 

[Hedges]

Short circuit current (Isc): 9.56A
Short circuit current (Isc): 0.30A

Since the two panels are same voltage, when connected in parallel even if the little one is shaded, no more current will flow backwards through its cells than what it would generate itself open circuit in full sun.

But, if it suffered an internal failure or wiring short, about 10A would be dumped into it. That's what the fuse is for.
Panels normally have a maximum fuse spec on their label. If this one doesn't use a fuse at least 156% of its Isc rating, so 0.5A. Could be up to about 0.75A or 1.0A. You will see the typical ratio of fuse rating to Isc on the larger panel.

Voltage mismatch with PWM wouldn't affect charging current, but while PWM is open allowing panels to rise to Voc, the paralleled panels would stop at Voc of the lower voltage panel, and it would carry all the current it produced plus the current from that voltage on the IV curve of the higher voltage panel(s). So it could carry considerably more current than it is supposed to, overheating and possibly being damaged.

 

[Sverige]

Thanks @Hedges that makes a lot of sense. So the two smaller 5 W panels can be paralleled with one another, then that chain should carry a fuse (2A?) before linking in parallel with the larger panel?

 

[gnubie]

Those panels are rated for 300mA, so 600mA for both. A 1A fuse would be OK between them combined and the common wire. I wouldn't go over 1.5A as you have to consider what happens if one faults and is able to draw 1.5A but not enough of a fault to draw enough current to quickly blow the fuse.

This is only to cover you for a gross fault in one of the tiny panels. With a 0.1V difference in Voc the current from the larger panel at 0.1V below its Voc is, I suspect, not sufficient to cause the smaller panel any damage. I have not squinted enough at the IV curve way down there but it's going to be minimal current. If someone does squint that hard or runs a formula to calculate the current that 0.1V shift would provide that might prove interesting.